Because of oil assay error, the trip reason of a combined cycle unit caused by the acceleration protection action is complex. Although, reliability inspection and field anti-interference performance tests were carried out on feedback device and signal cable shielding grounding, as well as foreign experts were invited for on-site analysis, it still failed to find the fault reason. Then, the oil line was checked and the fault reason were found that a large number of soft skins existed in the valve control oil filter. The fault phenomenon was eliminated by replacing the valve's filter. Finally, the experience and lessons of the fault analysis process are summarized tosuggest preventive measures for trip fault, which will provide references for professionals.